This invention relates to a closed loop servo-controlled stretch-forming machine of the type having two opposed clamping jaws. While the term "stretch-forming" is used in this application, the invention is intended to have application to any type of metal-forming machine wherein jaws are comprised of a number of adjacent grippers which are collectively curvable so as to more closely conform to the shape to be imparted to the metal.
The clamping jaws are formed of a series of hinged grippers move relative to each other in such a manner as to collectively form concave, convex or lazy-S curves. These opposed jaws are used to grip opposing ends of a metal sheet while the sheet is stretched into its yield state and while in that state is formed over a die. Each of the grippers is actuated against a mechanical or electrical stop by hydraulic cylinders so that the gripped sheet can be loaded flat, then caused to assume a contour roughly in the shape of the curved surface of the die. Thus, use of curved jaws in a stretch-forming machine saves material that would be wasted by the transition from the straight jaw's opening to the surfaces of the curved die.
For thin sheets, the curved jaws can apply a significant secondary forming action when forming parts such as aircraft fuselage parts by "gloving" the part while in the yield state over the die prior to the final longitudinal forming action.
Each of the grippers is controlled by a hydraulic cylinder, and the collective, accumulated motion of the hydraulic cylinders of adjacent grippers defines the curve of the jaw.
Several functions of such a stretch-forming machine have heretofore been controlled by various types of servo-feedback control devices. However, in prior art stretch-forming machines with curving jaws, the stroke of the hydraulic cylinder of each of the grippers is required to be mechanically adjusted and locked by a skilled set-up technicians. Such mechanical adjustment is disadvantageous for several reasons.
First, manual, mechanical adjustment is time-consuming and subject to trial-and-error adjustment and re-adjustment. Second, safety is compromised to the ex-tent that the technician is required to work in close proximity to heavy machinery and high hydraulic pressures. Third, creeping maladjustment may occur during machine operation requiring down-time to correct. Fourth, incorrect set-up may go unnoticed, resulting in wasted time and materials.
For these reasons, servo-control of the grippers is desirable to provide for a quicker, more precise machine set-up, to provide greater safety for machine technicians, and to provide constant feedback control based upon actual gripper and jaw positions during actual machine operation.